Power line communications (PLC) relates to the transmission of messages and data between a utility site and a customer site using the same lines over which electricity is conducted through the utility's power grid from the one point to the other. Communications are routed in both directions over the grid and can include instructions for configuring equipment (loads) at a site, data with respect to energy availability or usage, or reports as to the current status of operations at a site.
Typically, PLC occurs at frequencies of between 3 kHz and 500 kHz, or between 1.8 MHz and 250 MHz. In PLC systems, repeaters are often employed to extend the range of communications.
By contrast, Ultra Low Frequency (ULF) and Very Low Frequency (VLF) PLC occur at frequencies between approximately 500 Hz and approximately 10 kHz. In such applications, a voltage-controlled-voltage-source (VCVS) amplifier can be used as a source for ULF/VLF signals which are injected into and routed through power lines. An output of the VCVS amplifier is connected to a low voltage or LV power line which, in the United States, carries under 1000 v; i.e., typically 120 v. or 240 v.
The present invention is directed to a ULF/VLF coupler for use in such applications so to effectively isolate the VCVS amplifier from the 50/60 Hz waveform propagated through the grid while the ULF/VLF signals the amplifier supplies to the power line are minimally impeded. In addition, the power line impedance in the ULF/VLF bands is so low that it presents an extremely difficult load for VCVS amplifiers. The present invention addresses this problem such that the ULF/VLF coupler transforms power line impedance to a higher value which allows the amplifier to operate in a safe and stable region while effecting a reasonable power transfer from the amplifier to a power line.
Prior art couplers have typically been implemented using capacitive coupling or switched load transmitters. With regard to capacitive coupling, it has been found that for ULF and VLF band signals, the capacitor required may be too large to fit within a residential electric meter. With regard to switched load transmitters, it has been found they are limited as to the possible signals they can be used to transmit. For example, arbitrary waveform generation is not possible with these transmitters.
Finally, while coupling transformers are currently available, they are designed for operation only in narrowband and broadband PLC ranges not in the ULF/VLF ranges. Examples of such couplers are described in U.S. Pat. Nos. 6,407,987; 6,104,707; 5,717,685, and 5,559,377; as well as International Application Publication WO 2012/085059.